A spark gap device is an arrangement of electrodes between which a spark may occur. Spark gaps devices have applications as a pulsed heat source (automotive spark plugs as one example), a pulsed light source, and as a high voltage switch (pulsed power systems and lightning shunts for example). In these applications, the spark gap is designed to conduct once the applied voltage reaches a predetermined value.
Many spark gap device designs have been proposed. J. S. Whittier in U.S. Pat. No. 1,486,710 discloses an automotive spark plug with a hole drilled in the tip of one electrode. According to this patent, the hole causes the gap to produce a hollow cylindrical spark. The electrode face has sharp edges, and the sides have a relief cut into them to discourage carbon deposition. T. S. Schaub in U.S. Pat. No. 2,944,178 discloses an automotive spark plug having a hole drilled all the way through a fiat electrode. A second electrode has a matching coaxial hole. The two electrodes are otherwise fiat and sharp cornered. The two holes are stated to produce a jetting action of the fuel-air mixture, reducing carbon deposition. Lara et. al. in U.S. Pat. Nos. 4,015,160 and 4,023,058 disclose a spark gap for automotive applications having two electrodes with facing holes. The electrodes are either fiat or chamfered with straight edges and sharp corners. The facing holes are said to produce a hollow column spark. While these inventions strive to affect the shape of the spark produced when the gap breaks down, they do not address the voltage and timing characteristics of the breakdown.
The breakdown voltage of the spark gap determines the precise timing of the spark. Variability of the breakdown voltage causes imprecision in the timing of the spark. This variability, termed voltage jitter, can be up to 50% and is a major source of difficulty with prior art spark gap and electrode designs. The imprecision in spark voltage and timing due to excessive jitter is also a major source of other difficulties in systems using spark gaps. An example of this is the case where multiple spark gap switches are required to switch simultaneously. Even with the same applied voltage, jitter in the breakdown voltage of the gaps can cause the gaps to fire at different times and voltages. In addition to the associated timing problems, asynchronous spark gap firing can cause significant portions of the applied energy to be wasted. Inefficient energy use is a major inhibitor to the use of pulsed power in applications such as food irradiation, sterilization, and flue gas pollution control.
One solution to the problems posed by breakdown voltage jitter is to employ triggered spark gaps. In this form of spark gap, the applied voltage is kept below the breakdown voltage of the gap. At the appropriate time, the gas breakdown is initiated via external means (often a laser). Multiple spark gaps can be triggered nearly simultaneously in this manner, and the timing and voltage imprecision is much less. Triggered spark gaps, however, are much more expensive than self breaking spark gaps due to the external triggering means. The added complexity and expense of triggered spark gaps discourages their use in many applications such as automotive spark plugs,food irradiation, and flue gas pollution control.
A point-plane spark gap device is known to have low jitter. In this gap, one electrode is a point and the other is a plane. The breakdown characteristics of this idealized geometry are predictable, and lead to low jitter. This idealized geometry is unsuitable for many applications, however, since it breaks down at lower voltages for given electrode separations.